Through hull fittings on Aluminium sailboat

[Fore and Aft]

[DeValency]

Originally Posted by Wideocean7

Great debate. I should add that I do intend to venture to high latitudes. I didn’t make that clear when I originally posted, not understanding that it would make a difference I take on board comments that a 6mm aluminium hull might not be up “expedition” yacht standards, but that’s what I’ve got and can afford. (She was built in Vancouver with thicker bow plates and crash bulkhead). So I guess the question remains-are TruDesign fittings up for that or are the Groco fittings that way to go, now that I’ve said I will be venturing far north/south? For whatever reason I am very nervous about dissimilar metals, hence had been previously drawn to “plastic” fittings.

Great spirit!
TruDesign is based in NZ, that’s quite down south as far as I remember. You might want to give them a call.

IMHO, 6mm hull thickness is a plus as you’ll have more space to add a nice backing plate (aluminum? GRP?) to add even a better bonding and support from the inside, but I’m sure TruDesign will know all about it.

Happy expeditions! My mom says “take a sweater”

[double u]

"...For whatever reason I am very nervous about dissimilar metals..."

for VERY good reasons!
dissimilar Metals should be electrically insulated from one another in a marine environment, & that means "open connection" when tested with a VOM. This applies to above & even more so below water. Apart from owning an aluminium boat for 10 years & having been base manager on a charter base with 3 aluminium boats in the fleet the whole literature agrees that under no circumstances should stainless or worse bronze fittings be bolted/screwed directly to an aluminium hull, never ever.
Before my time at the base they used a brass-sleeved cutlass bearing on one of the aluminium boats (this was in Turkey & there wasn't anything else available at the time). We had the devils time getting the damned thing out a few years later...

[Manateeman]

No where in my boat is any stainless connected directly to the hull. If people would read my post again I specifically call out putting something, your choice, between the Groco adaptor plate and the reinforcement plate welded to the hull.
I also stated you can use Marelon thru hulls without nuts. And if you are still worried, you can CeraKote the Groco adaptor and the inside of the ball.
Seachests. You can make them any size or configuration you need. I like an opening somewhere large enough for my hand because stuff grows inside. I like a tee, no outside strainer, so you can poke out anything stuck in the thruhull.
You can put the strainer before or after the chest. You can weld nipples or use bulkhead fittings. You can add an electric/ manual valve with indicator so you can shut everything off any time you leave the boat. You can add a heating element on the exterior if you want to overwinter at the pole.
You don’t need to TIG the hull back up plate. You can do plexus ( just follow directions, rough up and acetone wash ) between aluminum and a Marelon thru hull and that job will go fast. You can tap the hull and use aluminum bolts set in plexus. You can have one standpipe welded in with a bolt flange on top and a blanking plate ready to go. Add an extra ear to the stand pipe flange so the blank plate is loosely bolted. You’ll get wet but you can swing the blanking plate into position without loosing it while you bolt it back down. Oil field trick.
My main engine and water maker have individual supplies. I have my strainer up stream of the chest but it’s your call. PM me and I’ll send you my email if you need more. Happy trails to you.
Captain Mark and his Miller Blue TIG manatees

[Shoalbay]

I owned a Brandlmayr 32 built in Al, for 29 years. The thru-hulls were nylon. The original valves were bronze; the 2 or 3 I added over the years were Marelon. No problems with valves.
A hole in the hull occurred due to a lead fish weight being dropped in the bilge. Luckily the issue revealed itself as a very localized weepy spot when the Salish Raven was on the hard.

[tomtriad]

Your query prompted me to look up poultice corrosion and I found the following sites:

https://stevedmarineconsulting.com/understanding-and-preventing-aluminum-corrosion/

https://stevedmarineconsulting.com/paint-and-aluminum-how-to-ensure-a-good-mix-2/

Very informative. You must read down the page on the 1st site anti where the discussion on aluminum starts.

[Dr.No]

Greetings! In the extensive refit of my Alubat Cigale 16, I'm replacing all the French ball valves on thru hulls. These seemed significantly inadequate; there was nothing that was a proper seacock. My approach starts with reducing at least half a dozen below waterline openings. This is primarily by going with a single fresh water head and a single black water tank, per Nigel Calder's excellent simplified system on page 653. With a Volvo saildrive providing engine intake cooling, there is no need to build a seachest to provide intake to the small owner's cabin air conditioner and a Spectra watermaker. Engine discharge is via the single factory welded tube. I like the Trudesign system, but decided on the Marelon Series 1 instead. These will use tapped G10 backing plates set with G/Flex along the lines of the Marinehowto article. They will be faired with epoxy putty, similar to the original build. Most of the above waterline discharges will use Series 1 seacocks, mostly because the original thru hulls needed replacing due to welding on a rubrail. The water tank hull vent will be rerouted to the head compartment, the fuel tank hull vent will be rerouted into a new radar arch. A major part of the refit is building two full width aluminum watertight bulkheads with custom doors from Freeman Marine. Cheers!

[Matt Johnson]

Originally Posted by Dr.No

Greetings! In the extensive refit of my Alubat Cigale 16, I'm replacing all the French ball valves on thru hulls. These seemed significantly inadequate; there was nothing that was a proper seacock. My approach starts with reducing at least half a dozen below waterline openings. This is primarily by going with a single fresh water head and a single black water tank, per Nigel Calder's excellent simplified system on page 653. With a Volvo saildrive providing engine intake cooling, there is no need to build a seachest to provide intake to the small owner's cabin air conditioner and a Spectra watermaker. Engine discharge is via the single factory welded tube. I like the Trudesign system, but decided on the Marelon Series 1 instead. These will use tapped G10 backing plates set with G/Flex along the lines of the Marinehowto article. They will be faired with epoxy putty, similar to the original build. Most of the above waterline discharges will use Series 1 seacocks, mostly because the original thru hulls needed replacing due to welding on a rubrail. The water tank hull vent will be rerouted to the head compartment, the fuel tank hull vent will be rerouted into a new radar arch. A major part of the refit is building two full width aluminum watertight bulkheads with custom doors from Freeman Marine. Cheers!

We got seriously close to buying a Cigale 14 a few months ago - my favorite monohull design - but the things you're now addressing were the few turnoffs that prevented us from going forward.

Good luck with the rebuild... it's a truly worthwhile boat to spend the time/money on.

Matt

[GordMay]

Some excellent articles on Metal Boats by Michael Kasten

“Corrosion Prevention For Metal Boats”
➥ Corrosion Prevention - Kasten Marine Design

Marine Metals Reference
➥ http://www.kastenmarine.com/_pdf/mbqMetRef.pdf

Corrosion, Zincs, & Bonding
➥ http://www.kastenmarine.com/_pdf/mbqCref.pdf

More articles from M. Kasten
➥ Articles on Yacht Design and Boat Building

[micknor51]

TRUDESIGN NZ through hull fittings and ball valves/seacocks. Made from glass reinforced nylon composite hence no corrosion problems. Certified with IMCI & Bureau Veritas.
I replaced all the seacocks on my steel yacht with the TRUDESIGN version several years ago and have had no issues with corrosion. Easily obtainable in Oz and quite reasonably priced.
https://www.trudesign.nz/
Good luck Michael👍😎

[Wideocean7]

Thanks all for feedback and website suggestions.

Michael-did you use backing plates when you installed Trudesign fittings. Trudesign “recommends” they be used but it actually seems that it would be essential on thin hulls as the thread on the thru-hull fittings doesn’t start until an inch or so. If you did use backing plates, what material? G10 has been suggested but it’s costly and I haven’t found a supplier in Australia. I can get hold of GPO-3 which has been suggested. Not sure if FRP will do the job. So my choices are between GPO-3 and heavily epoxy-coated marine ply. Interestingly, the cheaper option for the sizes I will be working with is GPO-3.

Cheers Andy

[mitiempo]

Originally Posted by Wideocean7

Thanks all for feedback and website suggestions.

Michael-did you use backing plates when you installed Trudesign fittings. Trudesign “recommends” they be used but it actually seems that it would be essential on thin hulls as the thread on the thru-hull fittings doesn’t start until an inch or so. If you did use backing plates, what material? G10 has been suggested but it’s costly and I haven’t found a supplier in Australia. I can get hold of GPO-3 which has been suggested. Not sure if FRP will do the job. So my choices are between GPO-3 and heavily epoxy-coated marine ply. Interestingly, the cheaper option for the sizes I will be working with is GPO-3.

Cheers Andy

GPO-3 is fine.

They even make whole boats out of it!

[Wideocean7]

Very good to know. Thanks Mitiempo.

[GordMay]

Originally Posted by Wideocean7

... G10 has been suggested but it’s costly and I haven’t found a supplier in Australia. I can get hold of GPO-3 which has been suggested. Not sure if FRP will do the job. So my choices are between GPO-3 and heavily epoxy-coated marine ply. Interestingly, the cheaper option for the sizes I will be working with is GPO-3.
Cheers Andy

It's an aluminum boat, so why not aluminum backers?
G10 is a FAR better product than GPO-3, which is better than plywood.
Plastics Sheet Comparative Data Chart
➥ Material Comparison Data Chart - Accurate Plastics - Yonkers, NY

[micknor51]

Andy,
I did not need to use through hull fittings since mine are welded steel with threads. However TRUDESIGN have a load bearing collar that can be used with the skin fittings. This should do the job I would think.
If you go to their website they have a product information sheet pdf available.
Michael